In recent years, in semiconductor devices, as the speed of an interface (I/F) part is increased and the voltage thereof is reduced, the transistor manufactured in the higher technology process tends to reduce the voltage that the transistor can handle. However, the majority of interfaces manufactured based on the standards not belonging to the high technology require high voltages. Because of this, when a high voltage output is produced in the output circuit formed by low withstand voltage transistors manufactured in the high technology process, a state where the withstand voltage is not sufficient is brought about.
Consequently, a high voltage output circuit is formed using low withstand voltage transistors by cascode-connecting the low withstand voltage transistors to disperse the voltage applied to the transistors. In such an output circuit, a drive signal the level of which is shifted is applied to the gates of part of the transistors and at the same time, a bias voltage is applied to the gates of the other transistors.
In the output circuit, the drive signal and noise from the output node affect the bias voltage and fluctuate the bias voltage. If the bias voltage fluctuates, there is a possibility that the voltage applied to the transistor exceeds the withstand voltage, and therefore, the transistor is destroyed.
In the case where a high frequency operation is performed in such an output circuit, the device size of PMOS transistors and NMOS transistors that appears from the output terminal is increased. Because of this, the capacitance between gate and drain of the transistor increases accompanying this and AC fluctuations at the output terminal largely affect the node of the bias voltage via the capacitance. Because of this, the bias voltage fluctuates and if the fluctuations are large, it is no longer possible to guarantee the withstand voltage.
Consequently, a bypass capacitor is connected between the signal line of the bias voltage and a reference voltage source (GND) and thereby the fluctuations in the bias voltage due to noise are suppressed. However, in general, the capacitor used within LSI increases the size of the LSI, and therefore, if a large-sized capacity is provided, the size of LSI is increased and if the size of the capacitor is reduced, the capacitance is reduced and it is not possible to sufficiently reduce noise.
Further, it is proposed to connect a capacitor between the signal line of the bias voltage and the signal line of the drive signal so as to suppress fluctuations in the bias voltage. However, in order to suppress the bias voltage, the capacitance value etc. is appropriately set.